1. Field of the Invention
The present invention is related to a process of impregnating a substrate having openings, in particular a continuous such substrate, for example a fabric such as cloth or mat made of various kinds of fibers with a resin composition in the form of liquid such as a vanish, an impregnated substrate with a resin composition using such a process and an article made of such an impregnated substrate, for example a so-called prepreg (i.e. pre-impregnate) and a laminated material.
As used herein, the resin composition is intended to mean a composition which comprises at least one component of a resin, and the resin composition may contain, in addition to such a resin component, other component(s) such as a solvent, a polymerizable monomer and/or a polymerization initiator. The resin composition in the form of liquid is intended to mean that such a resin composition is in a liquid state at a temperature at which the present process is carried out. The resin composition is preferably a solution, but it may be a dispersion or a suspension as far as no practical problem occurs. Concretely, the resin composition in the form of liquid herein includes, in addition to a resin composition which is liquid in its natural state (or a liquid resin composition), a composition prepared by dissolving such a liquid resin composition by a solvent and a composition which is prepared by dissolving a resin composition in the form of solid into a solvent. For example, a varnish (i.e. a resin composition dissolved into a solvent) and a so-called "resin varnish" (i.e. a resin composition which is inherently liquid even without dissolving into a solvent, for example a vinyl ester resin and a thermosetting resin composition comprising styrene) are herein examples of the resin composition in the form of liquid.
2. Description of the Prior Art
A prepreg (or a substrate which has been impregnated with a resin composition) which is used for the production of a laminate is generally produced by impregnating a substrate in the form of a layer having a fine open-cell structure with a resin varnish and drying and/or curing such a substrate. As the substrate, an elongated continuous substrate is usually used. The prepreg can be produced in a series of continuous steps of impregnating the continuous substrate with the resin varnish while supplying the substrate continuously, and further passing the impregnated substrate continuously through an oven so as to dry and/or cure the resin varnish.
As manners to impregnating a continuous substrate with a resin varnish as described above, there are a dipping method (in which the continuous substrate is dipped in the resin varnish in a vessel while supplying the substrate continuously), a showering method (in which the resin varnish is showered on the continuous substrate) and so on.
The dipping method has a problem that non-impregnated portions are very likely to be left in the interior of the substrate since the resin varnish tries to enter the interior of the substrate from both sides of the substrate so that air in the interior is sealed by the such resin varnish and the sealed air finally leads to bubbles. The showering method has a difficulty in high speed impregnation.
Then, a kiss-roll coating method has been proposed for example in U.S. Pat. No. 4,076,867. Referring to FIG. 6, the kiss-roll coating method is carried out by supplying a continuous substrate 1 as shown with an arrow while it is contacted with a periphery of a kiss-roll 11 which is placed in a varnish vessel 10 containing a resin varnish 2, so that the resin varnish 2 is coated on one side of the continuous substrate 1 by the kiss-roll 11. It has been stated that bubbles (or air) are unlikely to be sealed within the substrate 1 by the resin varnish 2 since the resin varnish 2 penetrates into the continuous substrate 1 from the one side thereof.
The kiss-roll coating method, however, has a problem that the continuous substrate 1 contacts with the kiss-roll 11 at a predetermined pressure (namely, a kiss pressure) so that the resin varnish is forced into the substrate, which causes insufficient impregnation leaving non-impregnated portions.
In connection with this problem, the following considerations are possible:
Referring to FIGS. 7(a) and (b) each of which schematically shows a cross-sectional view of a glass cloth 1 as a continuous substrate made of glass yarns 12, when the kiss-roll coating method is employed, one side (bottom surface) of the continuous substrate 1 contacts with the kiss-roll 11 at the kiss pressure so that the resin varnish 2 is passed through relatively wide openings between the yarns 12 (as shown with the arrows) in a short time to overflow onto the other side (top surface) of the continuous substrate 1, which seals the yarns 12 with the resin varnish 2 as shown in FIG. 7(a). Thus, air in small interstices (or openings) between filaments (not shown) which constitute the yarns 12 are confined in the resin varnish 2 and such air cannot escape from the surrounding resin varnish 2, and therefore the resin varnish 2 cannot penetrate into the interior of the yarns 12, which makes the non-impregnated portions left.
Further, since the continuous substrate 1 is continuously fed along its longitudinal direction, the substrate is under effects of its ambient circumstances, such as a wind due to the continuous feeding of the continuous substrate 1 and other air flow due to any other cause. For example, when the wind blows on the surface of the substrate as shown with the arrows in FIG. 7(b), a component having a lower boiling point which is contained in the resin varnish 2 tends to evaporate from exposed surface portions of the impregnated resin varnish 2 (namely, portions which contact with air). Then, the resin varnish of such exposed portions becomes more viscous, which suppresses the impregnation of the resin varnish into the interior of the yarns 12 by means of the capillary action so that non-impregnated portions are likely to be left. For example, with respect to a resin varnish 2 containing styrene monomer as a cross-linking agent, the problem tends to occur when some wind blows over the substrate 1 since styrene is ready to evaporate and a viscosity of the resin varnish becomes larger.